Diffusion confusion - what is the right diffuse shader yor which application?
Use diffuse for diffuse reflection, use glossy for glossy/specular reflection, and mix them because real materials also have multiple types of reflection at the same time. 

It is a common misconception to think that diffuse reflection is caused by the surface irregularities/bumps, but this is wrong, perfectly smooth surfaces still can have diffuse reflection. Actually diffuse reflection is caused by subsurface scattering, but the scattering distance for most materials and rendering scenarios is smaller than the pixel size, therefore luckily we can use the diffuse node instead of the slow subsurface scattering node.

For more details, see my answer to a similar question: What is the technical difference between a diffuse shader and a rough glossy shader?